A number of small desiccant canisters have been disclosed, which are formed from gas and liquid impermeable body portions onto which are secured one or more perforated end caps. These canisters generally contain a desiccant material which adsorbs moisture from the air as the air flows through the perforations provided in an end cap of the desiccant canister.
A common structure of such canisters is a one piece plastic body containing a cylindrical outer wall and a circular bottom wall, onto which is secured a cap containing a cylindrical outer wall and a circular top wall. The desiccant canister as disclosed in U.S. Pat. No. 5,759,241 utilizes a locking rib on the outer circumferential wall of the canister body which interacts with the cap to form a mechanical connection.
Mechanically assembled containers are sometimes problematic in that the mechanical connection between the canister body and the cap might not be strong enough to withhold a deformation of the canister under load conditions. Such load conditions might occur during use in a container filled with particulate matter or under exceptional circumstances, e.g. when the container is inadvertently dropped onto a hard surface. Load conditions leading to a deformation of the canister might also occur during the distribution of the canister in an automatic conditioning device. The resulting undesired opening of the container upon deformation has the consequence that its contents, e.g. dehydrating agents or oxygen adsorbents, might be introduced into the interior of the container and might contaminate goods contained therein, like drugs.
In order to solve this problem, it has been suggested to reinforce the stiffness of caps, e.g. by the provision of ribs. However, such solution can make it necessary or at least advisable to mount the cap to the canister body in a specific orientation, with a defined side of the cap oriented to the outside of the closed canister.
A different way to manufacture canisters is the assembly by means of the application of heat. When using a heat treatment, welding techniques have been previously suggested. An example of such a canister is disclosed in U.S. Pat. No. 5,824,140 describing a canister with an elongated hollow plastic body with two caps, which are fused to the ends of this plastic body. Specifically, it is suggested to use a manufacturing method including the steps of applying pressure and vibratory welding energy to the cap to form a fusion bond. However, the welding or any other comparable heat treatment to a porous membrane might be problematic because the membrane might be more sensitive to heat than the canister body. Accordingly, a heat treatment of a porous membrane might negatively influence its density, i.e. its permeability, and its behavior at the welded seams due to the degradation of the material of the membrane under an excessive thermal load.
As a further disadvantage, the functional material inside the canister might also be negatively affected by elevated temperatures so that a heat treatment is not feasible under certain circumstances. For example, gas uptake kinetic of chemical adsorbent may be catalyzed by heat. Further, heat exposure of adsorbents usually results in a loss of capacity.
A mechanical assembly followed by a welding of the contact region between the canister body and the cap involves a further manufacturing step and increases the production costs.
Finally, the use of perforated or micro-porous membranes results in a gas exchange kinetic which is nearly the same as the properties of the active material without the surrounding canister. In some instances, however, it is desirable to adjust the gas exchange kinetic.
A further disadvantage of perforated membranes is that powder active materials can contain particles or pieces of particles small enough to pass through the perforations, leading to a contamination of the content of the container in which the canister is introduced.